Owing, inter alia, to the brittleness of ceramic materials, the use of ceramic components composed of such materials is limited. It is therefore particularly important to improve, specifically, the mechanical properties such as strength, toughness, more favorable fracture behavior etc.
A process of the type described at the outset is disclosed by the Journal Nature, vol. 347, No. 6292, pages 455 to 467, Oct. 4, 1990: "A simple way to make tough ceramics". In this case, a composite material is produced in which first relatively thin portions are pressed out of ceramic material. These portions are formed with weak contact surfaces by covering them with, or bonding them to, graphite interlayers. The individual layers provided with the weak contact surfaces in this way are stacked one on top of the other and pressed together to form a composite material. Compared with materials of monolithic structure, the breaking strength is improved by a factor of 4 and the fracture energy is more than 100 times greater than in the case of monolithic products.
The improvement of the fracture toughness by the formation and disposal of weak contact surfaces between the individual layers of a composite material is intended to halt or deflect the propagation of cracks transversely to the main plane of extension of a composite material at each weak contact surface. The associated principles are described in detail in Refractory Materials, vol. 3, "Ceramic fibers and fibrous composite materials" by Rauch, Sutton and McCreight, Academic Press, New York and London, 1968.
US Pat. No. 3,007,222 discloses a process for the continuous production of sheet-type ceramic material, in particular in the form of tiles, heat protection layers and the like. Thus, for example, the parent body of a tile is consolidated by rolling the ceramic material. To form a layer of glaze, a further material is compacted between two rollers in their roller nip and the two layers formed in this way are brought together with low contact pressure by means of a further roller. This does not involve, however, the production of a composite material with improved fracture toughness, but it is intended to achieve as firm and permanent a joint between the layers as possible. The ceramic material for the layer forming the parent body of the tile is adjusted to a moisture content of between 8 and 15% by weight, the particle size being less than 75 .mu.m